1. Field of the Invention
The invention relates to a method of adjusting a disc brake in a squirrel cage motor comprising a stator, a rotor arranged on a shaft of the motor and comprising a deflector for deflecting part of the magnetic field of the stator to a stray flux of the magnetic field of the rotor in the direction of the shaft, the motor further comprising an armature arranged on the shaft, a flexible element arranged between the deflector and the armature for pushing the armature away from the deflector, at least one brake disc supported to a motor frame via at least one-slot, at least one trigger gear arranged on the shaft and a shaft bearing, in which method the size of an air gap between the deflector and the armature is adjusted when the motor is idle.
The invention further relates to an arrangement for adjusting a disc brake in a squirrel cage motor, the arrangement comprising a rotor arranged on a shaft of the motor and comprising a deflector for deflecting part of the magnetic field of a stator in the motor to a stray flux of the magnetic field of the rotor in the direction of the shaft, the arrangement further comprising an armature arranged on the shaft, a flexible element arranged between the deflector and the armature for pushing the armature away from the deflector, at least one brake disc supported to a motor frame via at least one slot, at least one trigger gear arranged on the shaft and a shaft bearing, whereby, when the motor is turned on, the stray flux of the magnetic field of the rotor shorts via the armature generating a magnetic axial force between the deflector and the armature in the direction of the shaft, the force attracting the armature against the deflector thus opening the brake formed by the armature, the brake disc and the trigger gear, and when the motor is turned off, the axial force between the deflector and the armature ceases, whereby the flexible element pushes the armature away from the deflector thus activating the brake and stopping the motor, whereby an air gap forms between the deflector and the armature.
2. Description of the Prior Art
Squirrel cage motors employ a disc brake structure in which part of the active length of the motor rotor forms a deflector that affects the magnetic field of the motor stator and turns part of the magnetic field of the stator in the axial direction of the motor. Said magnetic field in the axial direction of the motor, i.e. the stray flux of the magnetic field of the rotor, shorts via the armature of the motor generating a force between the deflector and the armature in the axial direction of the motor, which force tends to pull the armature against the deflector. This movement opens the brake of the motor, which is closed when the motor is idle. Then turning off the motor, the magnetic force between the deflector and the armature disappears, whereby a spring arranged on the motor shaft, between the deflector and the armature detaches the armature from the deflector and pushes the armature against the friction elements of the brake structure. Thus the brake is activated and stops the rotor. An advantage of the brake structure is that no separate winding or separate control is needed for its implementation. However, the problem is that as the friction elements of the brake wear, the distance between the deflector and the armature increases, finally increasing so large that the axial force between the deflector and the armature is no longer capable of attracting the armature against the deflector. This means that the brake is not opened, but the motor has to revolve against the torque of the brake, which may lead to burning of the motor winding. Because of the risk of burning of the winding, the air gap between the deflector and the armature has to be maintained sufficiently small, wherefore the air gap between the deflector and the armature has to be checked and/or adjusted every so often or at given intervals.
A known way to adjust the air gap between the deflector and the armature is to arrange shims between the bearing that is locked to the motor shaft with a safety ring on the side of the brake and the trigger gear locked to the shaft with cogging, the amount and thickness of which shims vary. However, the method is extremely laborious and time consuming, since a suitable combination of the amount and thickness of the shims has to be found, and the motor has to be disassembled in order to adjust the air gap.
In another known solution, the motor bearing on the side of the brake is placed in a separate bearing housing, which, in turn, is arranged with threads in a bearing plate. In this case the air gap between the deflector and the armature is adjusted by turning the bearing housing with respect to the bearing plate and by locking the turning of the bearing housing after the adjustment. The problem in this solution is that the clearances between the threads of the bearing housing and the bearing plate affect both the air slot between the stator and the rotor and the air gap between the deflector and the armature. The air gap of the brake cannot either be measured upon assembly of the motor. Furthermore, the change in the shaft bearing point due to the turning of the bearing housing may affect the operational reliability of the motor.
EP 0 742 633 discloses an electric motor comprising a deflector that is slidingly coupled to an axial shaft of the motor and deflects part of the magnetic field of the stator to an axial magnetic field. An armature is further arranged on the motor shaft, and the brake element of the motor is fixed thereto. A helical spring in interposed between the deflector and the armature. When the motor is activated, the force of the axial magnetic field attracts the armature towards the deflector, thus opening the brake. For changing the size of the air gap between the deflector and the armature, the motor shaft is provided with two bores. One of the bores extends diametrically through the shaft and is provided with a pin that is fixed to the deflector at its both ends. The second bore is in the axial direction of the motor and has an adjusting rod fixed at its one end to a pin arranged in the diametric bore of the shaft and at its other end to a threaded bolt, which is turned to move the adjusting rod in the bore so as to change the size of the air gap between the deflector and the armature. U.S. Pat. No. 4,496,864 discloses the same type of brake structure for an electric motor, wherein the size of the air gap between the deflector of the motor rotor and the motor armature is changed by moving an adjusting nut disposed in an axial bore. The movement of the adjusting nut is transmitted as a movement of the armature by means of a pin disposed in a bore extending radially through the shaft. The brake operates in principle in the same way as was described in connection with EP 0 742 633. The drawback in the solutions of both publications is that they are complex and expensive to manufacture. Axial and radial bores supplied in the motor shaft are difficult and expensive to make and they weaken the structure of the shaft and, consequently, the service life of the motor. Furthermore, the brake structures of the publications cause much work during assembly.
The object of the present invention is to provide a simple, fast and precise manner of adjusting the disc brake of a squirrel cage motor.
The method of the invention is characterized by adjusting the size of the air gap with at least one adjusting piece movable in the longitudinal direction of the shaft between a bearing surface in an inner ring of the bearing and the shaft in such a manner that the movement of the adjusting piece makes the armature move in the longitudinal direction of the shaft thereby changing the size of the air gap between the deflector and the armature.
The arrangement of the method is characterized in that the arrangement also comprises at least one adjusting piece movable in the longitudinal direction of the shaft between a bearing surface in an inner ring of the bearing and the shaft, and means for moving said adjusting piece in such a manner that the movement of the adjusting piece causes a movement of the armature in the longitudinal direction of the shaft thereby changing the size of the air gap between the deflector and the armature.
According to the essential idea of the invention the squirrel cage motor comprises a stator and a rotor arranged on the motor shaft and comprising a deflector that deflects part of the stator magnetic field to a stray flux of the magnetic field of the rotor in the axial direction of the motor. The squirrel cage motor further comprises an armature arranged on the shaft, a flexible element arranged between the deflector and the armature for pushing the armature away from the deflector, at least one brake disc supported to the motor frame via at least one slot, at least one trigger gear arranged on the shaft and a shaft bearing, such that, while the motor is idle, an air gap forms between the rotor deflector and the armature. For the adjustment of the size of the air gap, i.e. the disc brake of the squirrel cage motor, the squirrel cage motor further comprises at least one adjusting piece movable in the longitudinal direction of the shaft between the bearing surface of the inner ring of the bearing and the shaft, and means for moving the adjusting piece so that the movement of the adjusting piece causes a movement of the armature in the longitudinal direction of the shaft so as to steplessly change the size of the air gap between the deflector and the armature. According to a preferred embodiment of the invention the end of shaft comprising the brake disc and the trigger gear is provided with threads and an adjustment nut moving in the longitudinal direction of the shaft and adapted to the threads so that the turning of the adjustment nut causes a movement of the adjusting piece in the longitudinal direction of the shaft. According to another preferred embodiment of the invention, the wearing parts of the brake of the motor are placed in a different space than the winding of the motor.
An advantage of the invention is that the adjustment of the air gap between the deflector and the armature is simple and fast since the air gap can be adjusted as desired without disassembling the motor. Furthermore, when the air gap is adjusted, the bearing point of the shaft does not change, and therefore the operational reliability of the motor remains high. A further advantage is that the motor shaft does not have to be provided with longitudinal and radial bores, which weaken the structure of the motor. The placement of the wearing parts of the brake structure in a different space than the motor winding prevents the wear products of the brake from damaging the stator winding. The braking torque may also be easily increased simultaneously by increasing the number of friction surfaces in the brake.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.